The manufacture of integrated circuits involves a large number of steps in which a silicon wafer is coated first with a primary coating, e.g., aluminum and then a photoresist coating. It is then exposed to circuit patterns previously formed on a mask. One such system exposing a silicon wafer to the circuit patterns on a mask is described in U.S. Pat. No. 4,011,011 entitled Optical Projection Apparatus and assigned to the Assignee of the present invention. After exposure the silicon wafer is developed to remove portions of the photoresist leaving an outline of the circuit patterns from the mask. Next, the primary coating of the wafer not covered by the photoresist is etched to expose the silicon wafer surface in patterns determined by the remaining photoresist. The remaining photoresist is then stripped. At this point the silicon may be processed further, e.g., doped by injecting materials into the surface of the silicon wafer to form portions of electrical components such as transistors, diodes etc.
This process is repeated a number of times until complete circuits are built up on the wafer which is then sliced along rows and columns to form a number of identical, individual integrated circuits.
A critical step in this fabrication process is that of etching. In integrated circuit fabrication etching is a technique for the selective removal of material from the wafer. The areas etched are those lines formed by the removal of photoresist.
One method of etching known as wet processing involves the use of chemicals in liquid form which remove the oxide layer on the silicon wafer from areas not covered by the photoresist. This method is highly isotropic.
Another method of etching known as dry processing involves the use of gas in an evacuated chamber containing the wafer. The gas is ionized by an R F source to create ions which attack the coating on those wafer areas or lines not covered by photoresist. Dry processing is less costly, more selective, and easier to control than wet processing. In addition, dry processing is highly anisotropic, has higher yield and is more susceptible to automation than wet processing.
A problem associated with both methods of etching wafers is atmospheric contamination of the photoresist coating on the wafer. Exposure to atmospheric moisture causes a thin top layer of the photoresist coating to deteriorate. This moisture can react with the etching gases or liquids to give undesirable effects.
This problem may be overcome by pretreating the wafer in vacuum immediately prior to etching and thus without further exposure to the atmosphere perform the etching step.